1. Field of the Invention
The present invention relates to a single photon generating apparatus.
2. Description of the Related Art
With explosively spreading Internet and practically beginning electronic commerce, encryption technique is needed for reservation of secrecy of communication and personal identification. As the encryption technique widely used, there are a common key method such as a DES encryption and an open key method such as a RAS encryption.
However, the conventional encrypting technique is based upon xe2x80x9cquantitative securityxe2x80x9d. Therefore, these encrypting method are always threatened by progressing computer hardware and decrypting algorithm. Accordingly, if a theoretically secure encrypting method is put into practice in various fields, where extremely high security is required, such as transactions between banks and information concerning military affairs and diplomacy, the impact thereof will be great.
Studies of quantum encryption have been extensively conducted since the proposal of a specific protocol made by Bennett, Brassard et al. in IEEE International Conference on Computers, Systems, and Signal Processing, (Bangalore, India, p. 175, (1984)). As an encrypting method in which unconditional security has been proved in information theory, there is a one-time pad method. The proposed quantum encryption protocol provides a method of securely delivering an encryption key used in the one-time pad method. Since the security of quantum encryption is guaranteed by physical rules, an ultimately guaranteed security can be accomplished which does not depend upon performance limitations of computers. The quantum encryption bases its security upon the fact that a wiretapper cannot know the state of one photon completely. Therefore, it is required to transmit information of one bit by use of only one photon in order to guarantee the security of quantum encryption. That is, it is important for a quantum encrypting apparatus to generate a single photon securely at a predetermined time.
It is known that the use of a pair of entangled photons is effective for quantum encryption. For example, Briegel et al. have reported that a quantum state can be relayed by using a pair of entangled photons (Physical Review Letters, Vol. 81, p. 5932, 1998). Usually, for generating a pair of entangled photons, a method called Parametric Down Conversion is used. In this method, a pair of photons having one half of energy of incident light into a nonlinear optical crystal are generated.
However, many combinations of photons are possible concurrently within a range satisfying the principle of conservation of energy and number of waves in this method. Therefore, the generation of a pair of photons to be obtained is probabilistic, and moreover, the generation probability is very low in the order of one-ten thousandth. Thus, this method is not practical.
In contrast thereto, a pair of entangled photons can be obtained by connecting the outputs of two single photon generating apparatus to a quantum gate such as a control NOT gate. In this method, a pair of entangled photons can be obtained efficiently. However, two apparatus, that is, a single photon generating apparatus and a photon control unit (quantum gate), are newly required.
In order to realize a single photon generating apparatus applicable to quantum encryption, the following two factors are required: (1) Only one electron is excited, and the excited state is maintained till the electron emits a photon and returns to the ground state thereof; and (2) the photon is taken out from the apparatus at a predetermined time.
In the report by De Martini et al. (Physical Review Letters, Vol. 76, p. 900, 1996) and in the report by Law and Kimble (Journal of Modern Optics, Vol. 44, p. 2067, 1997), it has been proposed to control the intensity and time period of excited pulse light in order to excite a single electron.
Moreover, Susa has proposed a single photon generating element in Japanese Laid Open Patent application (JP-A-Heisei 4-61176), in which electrons are injected one by one into a semiconductor active layer by using a phenomenon in which the tunneling of the electron is prevented due to the change of electric field caused by the single electron existing in a semiconductor thin film, that is, by using so-called a coulomb blockade.
Kim et al. have reported a single photon generating element in which electrons are injected one by one into a semiconductor active layer by means of a method called turn style based upon a principle similar to the above technique (Nature, Vol. 397, p. 500, 1999).
However, in the single photon generating methods of Susa and Kim et al., the generation of a photon is a probabilistic event caused in a time period determined based on the lifetime of the electron in the exited state. The generated photon is immediately emitted out of the apparatus. Therefore, the time period during which the photon is emitted is relatively wide in the order of nanoseconds. Since generally used photon detectors can analyze such time period, these methods are insufficient to emit the photon out of the apparatus at a predetermined time.
On the other hand, it is known that the light emission efficiency increases by using a micro resonator having a high Q-value as proposed by De Martini as well as Law and Kimble. By utilizing this effect, the time period of light emission can be narrowed. However, the generated photon cannot easily leave the resonator and leaks out of the resonator over a long time. Therefore, the time when a photon is emitted out of the apparatus cannot be determined.
For example, Collot et al. have reported that a high Q-value of 2xc3x97109 is obtained by using a whispering gallery mode resonator (Europhysics Letters, Vol. 23, p.327, 1993). Since the frequency of light is in the order of 1015 Hz, the lifetime of the photon in this resonator is almost a microsecond.
In conjunction with the above description, a single photon generating apparatus is disclosed in Japanese Laid Open Patent Application (JP-A-Heisei 4-61176). In this reference, a first semiconductor is sandwiched between second and third semiconductors. The first semiconductor has electron affinity larger than the second and third semiconductors and a band gap energy smaller than the second and third semiconductors. P-type or n type impurity is doped in either one of the second and third semiconductors and the other semiconductor is not doped. The other semiconductor has the thickness in a range of 1 nm to 20 nm.
Therefore, an object of the present invention is to provide a single photon generating apparatus which can emit a single photon out of the apparatus at a predetermined time.
In an aspect of the present invention, a single photon generating apparatus includes an optical waveguide, an active medium section and a resonator section. In the active medium section, a single electron is excited in response to application of exciting energy, and a single photon is emitted from the electron. The resonator section optically resonates with the active medium section, holds the photon emitted from the electron in the resonator, and transfers the held photon to the optical waveguide in response to a first control signal.
The application of the exciting energy may be achieved by application of a first light pulse. In this case, it is preferable that the first light pulse has a duration time shorter than a recombination time of the electron.
Also, the active medium section may include a semiconductor substrate, a quantum dot, a cap layer and first and second electrodes. The quantum dot is formed on the semiconductor substrate, and the electron is excited in response to the application of the exciting energy. The cap layer is provided between the quantum dot and the resonator section and optically separates the quantum dot from the resonator in response to a second control signal. The first electrode is formed on the cap layer apart from the quantum dot in a horizontal direction. The second electrode is formed on a surface of the semiconductor substrate opposite to the quantum dot in correspondence to the first electrode. The first light pulse is applied to the quantum dot from a region where the second electrode is not formed.
In this case, it is preferable that the cap layer has a thickness equal to or less than xc2xd of a wavelength of the photon.
Also, the single photon generating apparatus may further include a first applying section which applies a first electric signal between the first and second electrodes after the first light pulse is applied such that the quantum dot is optically separated from the resonator section. In this case, the first applying section may apply the first electric signal between the first and second electrodes in response to application of a second light pulse to the first applying section.
Also, the resonator section may include a resonator and a connection member. The resonator optically resonates with the active medium section, and holds the photon therein. The connection member is provided between the resonator and the optical waveguide and passes the photon from the resonator to the optical waveguide in response to a second control signal.
In this case, it is preferable that the resonator has a Q value equal to or larger than 104. Also, the resonator is preferably formed of either semiconductor, dielectric substance, or a local defective portion of photonic crystal where periodicity is disturbed.
Also, the resonator preferably has a spherical shape which has a characteristic length in a range from xc2xd of a wavelength of the photon to 100 times of the wavelength of the photon.
Also, the connection member may be made of electrooptic effect material, and the connection member changes a refractive index in response to the second control signal such that the photon is passed from the resonator to the optical waveguide.
Moreover, the single photon generating apparatus may further include a second applying section which applies a voltage signal as the second control signal to the connection member. In this case, the second applying section applies the second control signal to the connection member in response to application of a third pulse signal. Also, it is preferable that a response time of the second applying section is shorter than a lifetime of the photon.
Also, the optical waveguide is preferably arranged such that optical coupling between the optical waveguide and the resonator section in a whispering gallery mode is minimum.
Also, the active medium section may include a quantum dot in which the electron is excited in response to the application of the exciting energy such that the photon is emitted, and the resonator section may be formed by a first region of a photonic crystal other than a second region. At this time, openings are arranged in triangular lattice in the second region of the photonic crystal, and the quantum dot is arranged in the first region.
Also, the single photon generating apparatus may further include a light pulse applying section which applies a fourth light pulse as the first control signal to the resonator section. The resonator section changes a refractive index in response to the fourth light pulse such that the photon is connected to the optical waveguide.
In another aspect of the present invention, a method of generating a single photon, is attained by (a) exciting a single electron in a quantum dot; by (b) confining a single photon generated from the electron in a resonator; and by (c) leading the confined photon into an optical waveguide.
In this case, the electron is excited in the quantum dot in response to application of a light pulse. Also, it is preferable that the quantum dot is optically separating from the resonator after the photon is emitted from the electron.
Also, the resonator may be optically connecting with the optical waveguide in response to a control signal when the photon is confined in the resonator. In this case, a refractive index of a connection member which is provided between the resonator and the optical waveguide is changed in response to the control signal such that the connection member is transparent to the photon. The control signal may be a light pulse signal.
Also, a refractive index of the resonator may be changed in response to the control signal such that the photon is connected to the optical waveguide.